Beatrice is an artificial spirit crafted over four centuries ago by Echidna, the Witch of Greed. Created to safeguard Roswaal Mathers' Forbidden Library, she was bound by contract to wait indefinitely within it for an unspecified "that person," who would grant her salvation and release. This enforced isolation fostered profound loneliness and emotional detachment, leading her to adopt a haughty, dismissive attitude toward library seekers or those coveting her power as a Great Spirit. Her sole early connection was with Ryuzu Meyer of Echidna's Sanctuary; Ryuzu's traumatic sacrifice during Hector's attack left Beatrice emotionally scarred for centuries.

Physically resembling an 11–12-year-old girl, she has cream-blonde hair in twin drill-shaped pigtails with pink-dyed tips, butterfly-patterned sky-blue eyes, and typically wears ornate frilly dresses. She speaks distinctively, often ending sentences with phrases like "I suppose" or "in fact." Her initial personality blended intellectual pride—especially regarding her mastery of Yin Magic—with deeply guarded vulnerability. She exhibited tsundere tendencies, masking softness with sarcasm or irritation, and displayed mischief through magical pranks. Centuries of solitude eroded her faith in Echidna's promise, culminating in suicidal despair by Arc 4.

Her abilities center on Yin Magic: spatial manipulation (Door Crossing for teleportation/interdimensional travel), time manipulation (Minya stakes freezing/shattering time to bypass regeneration, Murak altering gravity), sensory disruption (Shamak variants inducing darkness or mental fog), and high-level offense (Ul Shamak creating black holes). She also removes curses, heals, and creates barriers. Historically immense within the Forbidden Library, her power rivaled Roswaal's but demanded vast mana. Originally sustained by draining mansion residents via the library, her limitation worsened after contracting with Subaru Natsuki. Their pact restricts her mana sourcing exclusively to him, preventing atmospheric absorption or self-restoration; exhausting her mana risks physical dissolution.

Her dynamic with Subaru began antagonistically; he irritated her by ignoring rules and treating her casually, yet his persistence gradually broke her emotional barriers. Though she suspected he might be "that person," his repeated denials crushed her hope. When the mansion burned in Arc 4, she resolved to die inside the library. Subaru refused to abandon her, shattering her despair by vowing to replace "that person" and create memories outweighing eternity. This convinced her to leave, forging a lifelong contract. Post-contract, her demeanor softened significantly. She developed fierce loyalty, referring to him by name—a rarity indicating deep affection—and became openly reliant, often holding his hand or sharing his bed like a sibling. They collaborate on original spells, like one allowing partial existence outside reality.

Her relationships expanded: she maintains a respectful but distant tie to Roswaal, rejecting familial labels. Her bond with Echidna’s other artificial spirit, Puck, is sibling-like, though she envied his freedom. After contracting Subaru, she reconciled with Emilia, transitioning from avoidance to mutual support and using her name. She playfully teases Otto Suwen and takes a protective stance toward allies like Rem. Her combat role involves strategic support, such as healing during battles against foes like the Great Rabbit or Regulus, though mana constraints sometimes limit her effectiveness.

Her later development emphasizes increased socialization within the Emilia Camp, trust in Subaru’s decisions—like accepting Spica—and resilience during separations, including his months-long displacement in Vollachia. She expresses concern for his safety due to his deteriorating Gate, while he worries about her mana supply. Her actions consistently prioritize his well-being, such as shielding him from psychological trauma inflicted by enemies mimicking Rem.